This invention is concerned with pull-type blind fasteners, that is to say with fasteners of a kind comprising a push-part having a head for engagement with the exposed side of a workpiece and a pull-part which passes through said head and projects as a pulling stem therefrom, a shank provided by one of said parts being effective when the shank has been inserted through aligned holes in a workpiece and the stem is pulled to engage the workpiece at the blind side to clamp it against said head. The fastener also has means for effecting inter-engagement between the two parts to lock them in set relationship and to hold them together on removal of the pulling stem.
Metal pull-type blind rivets have been known for many years, one brand having manufactured and widely marketed by the company assignee of the present invention under the registered trademark "POP." Such rivets have a rivet body, or "push-part," and a mandrel or "pull-part," which may be pulled right through the rivet body, or broken to leave a head part in the set rivet, or may be subsequently trimmed to remove the stem projecting from the rivet head after setting. Metal pull-type blind rivets enable reliable and permanent riveted joints to be made in workpieces comprising sheet metal panels, for example, to which access is available from only one side, and may be designed not only to give strong joints with the parts of the workpiece tightly clamped together but also to afford a high degree of pulling together of the parts of the workpiece if, as is very common, there is a small gap between them at the beginning of the riveting operation.
Among the wide variety of applications for which blind rivets are used are some for which metal is regarded as undesirable, if not altogether unsuitable. Electrical insulation, lightness in weight, choice of color, for example, are factors which have led to the introduction of plastic blind fasteners. However, such plastic fasteners, especially those which resemble pull-type blind rivets in appearance, usually suffer from a very low ability to pull parts of a workpiece together, and very low residual clamping force in the set fastener.
An example of a two-piece plastic pull-type blind fastener which has been proposed is one comprising a part which is pushed, i.e. a "push-part," and which has an annular head with a hollow shank projecting forwardly from it for insertion in a hole in a workpiece, and a pull-part accommodated in the shank and having a stem projecting from the head of the push-part so that it can be pulled. The push-part of this fastener has two diametrically opposed slots running along an intermediate portion of the shank between the head and an annular wall at its tail end and the pull-part extends beyond the shank and there, has a head of the same diameter as the outer diameter of the annular wall so that, when the pull-part is pulled, the head abuts the wall and the intermediate portion of the shank splays out like two elbows to engage the blind side of the workpiece. Teeth may be found on the pull-part so that they dig into the material of the head of the push-part and lock the two parts of the fastener together in set position more firmly than relying on friction, the two parts thus remaining in their set condition when the portion of the pull-part projecting from the head of the push-part is trimmed off.
The fastener can be made by injection molding the pull-part first and then the push-part over it. Whereas this particular example of a plastic blind fastener has generally adequate strength and an ability of pull parts of a workpiece together, the range of workpiece thickness with which it can be used is limited and it requires relatively complex tooling for its manufacture.
It is an object of the present invention to provide an improved blind fastener which can be made economically of plastic material and which will yet exhibit a high degree of ability to pull together parts of a workpiece, and hold them tightly clamped after setting.